Scaling/Peeling Concrete Surface? 2

[jryooper]

I am running into one and two-year old concrete slabs (interior and exterior) that have been suceptable to "Freeze/Thaw" conditions that are flaking/peeling the top 1/8" (cream) off after winter's weather. Not all slabs that have been poured, but some. Different concrete contractors have placed and finished these slabs so the finishers haven't been a common denominator. These slabs weren't poured at a slump over 5-5 1/2". Weather has been different, but no rain or water was ever applied. De-icing salts were never applied directly, but I'm sure some salty water dripped from vehicles from time to time. Sealers were applied to several of these but not all (after a 28 day cure time). The Redi-mix plants claim proper air was entrained. FYI: Two of the plants both used Laf**ge and a third used Cem*x. The course aggregate hasn't "popped" at all. Just the cream in various spots. Some slabs were finished by only screeding with a wood 2X4 and bullfloated and then broomed, while others were screeded with an aluminum Power/vibra screed then mag. bullfloat, then steel fresno and then broomed (if exterior) or power/hand trowel (if interior). What's going on? Thanks to all and sorry about the novel.

 

[Ron]

jryooper....what you are describing is generally a finishing problem. Likely that there was a lot of bleed water and it's possible that the finishers might have done some "dry shaking", using plain cement to dry it up, though excessive bleed water will cause that by itself.

If the surfaces had been hard troweled, I would have suspected "blistering", but it sounds more like excessive scaling.

A petrographic examination of several cores will tell you the probable cause of the issue.

 

[jryooper]

I've actually overseen these projects and each one was done by experienced professional masons. For some reason (whether it be the humidity, slump, or redi-mix) we don't get much bleed water (if any). There was no "dry shaking" to speak of. I would still like to get a petrographic examination done, though and have been considering it for a while. Is there anyone you could recommend? Thanks a lot for the input and any more information provided.

 

[JAE]

Agree with Ron - blistering would be due to hard troweling I'd think. Perhaps it is as simple as the finisher not waiting long enough for the bleed water to rise and water was trapped just below the surface. But the fact that no aggregate was included in the popped up spalled pieces suggests the dry shaking Ron suggests.

jryooper - can you post some pictures?

 

[concretemasonry]

It is definitely a timing/procedure problem. The mix is probably uniform, but no specifications can accurately control the required timing of the final finishing steps ans methods. If you can find a spec writer than can do it, he will be able to walk on water and then be sued.

The 28 day cure time is variable depending on the temperatures, exposure and conditions of the concrete and not just the air. The 28 days referred to is for lab cylinders and idealists.

The fact that it is occurring on just some slabs indicate that the problem is isolated to different panels. Unless the panels were very large, it is likely that the concrete in several panels may have been from the same load/batch, so placement and especially finishing was the variable.

If you really want a good analytical analysis, we have a very good local testing lab that works arm-in-arm with an engineering firm that has done significant interpretation of thin slices of concrete under an strong microscope to determine the failure source regarding similar surface problems (and deeper problems that may occur later). - I can provide the contacts.

Incidentally, a person that finishes concrete is a "cement finisher" and not a "mason", which is a totally different trade. - just ask a contractor or a union.

Dick

 

[Ron]

JAE is correct about trapping bleed water. Excellent point.


Three good petrographic labs....

Construction Technology Laboratories in Skokie, Illinois (

http://www.ctlgroup.com)-Multiple

offices

Simpson, Gumpertz, and Heger in Boston (Waltham, Massachusetts) (

http://www.sgh.com)-Multiple

offices

Wiss, Janney, and Elstner-Chicago
(

http://www.wje.com)-Multiple

offices

I don't have a specific contact at CTL, but at SGH you can ask for Paul Kelley (Boston office) or at WJE check with Bill White in their Atlanta office.

 

[jryooper]

Why did it take two winters before anything happened? Winters are about the same here with freeze/thaw conditions mainly in the spring (when this peeling has been noticed). Like I say "I was on site for these pours and supervised" (not to say everything I do is a written procedure), but when the bleed water is gone "it's gone" and no two slabs act the same of course and there is definately no book that tells you exactly how long to wait (in time)before bullfloating, troweling, screeding, brooming etc... Again it is happening around here in approximately a 100 mile radius with the same "cement finishers" (sorry for the wrong terminology earlier)that have been doing this for 30+ years without ever seeing this before. Thanks for the input so far. I talked with CTLGroup last year and it was a bare minimum of $12,000 to core and test three slabs....How could I possibly recover that? We are small construction market and the economy sucks.

 

[Ron]

With each additional amount of information, it becomes more apparent that you have issues that are not likely to be solved without a site visit and/or coring and petrography.

Please keep in mind that the comments from your finishers are very common, mostly because unless there is economic loss sufficient to bring them in, when they walk from the job they never see how it performs.

I have seen the problems you describe on numerous occasions. I have seen it result from freeze-thaw cycling, from dry shaking, from retempering the mix, from wind conditions during placement and finishing, from mix design problems, and from combinations of those conditions.

Based on your description of the timing, it is likely that your entrained air content in the surface paste was reduced for some reason and then freeze-thaw took a few cycles to work on it. Just conjecture, though, not having seen photos or the site.

As for the fee estimate by CTL, it seems a bit high, but then I don't know what they proposed.

In general, we charge from $800 to $1200 per core for petrography, depending on what is needed (air content, water-cement ratio, etc.). As for a field visit and coring, it depends on where it is relative to our office. Coring runs about $250 per core, locally. For evaluation and reporting, you're probably looking at somewhere around $2500 to $3000. Given all that, a fee in the range of $5000 to $8000 would be about right, plus travel if it is a remote site (remote to one's office, not remote from civilization!).

You might check around a bit more or if you will tell me your area, I might know someone you could contact. I am not offering our services, I just included that information as a point of reference for you. It would not be appropriate to offer services in the forums, and resultingly, I would not accept an engagement from this even if in my geographic area, but would be glad to help you find someone.

 

[jryooper]

Ron,

Thanks a lot for all of your input. I greatly appreciate it. If it is possible (within this site) to give me an email address I can contact you with, I will reveal my location and get you some pictures. If not I'll continue to try to explain the sitaution as best I can. Thanks again. I have one of these slab in my driveway and will get some pictures today and try to post them so you can see.

 

[Ron]

jryooper...contact Eng-Tips and tell them you want to contact me directly. They will facilitate by either giving me your email address or giving my address to you.

Ron

 

[jryooper]

Thanks Ron. I'll do that. I still haven't got those pics uploaded yet, but they will be within the next day or two.

 

[emmgjld]

Agree with all that has been said.

A similar and increasing problem we have encountered in western Colorado (USA) in the last 5 to 10 years has usually involved concrete placed in the late Fall. Some of the scaling was not severe for 3 to 5 years, indicating a problem near the surface of the concrete mass which was 'waiting' for the proper final conditions to expose the existing weakness. A lot of testing data is available and a fair amount of petrographic analysis, with 'inconsistent' conclusions. I say 'inconsistent' not in a derogatory manner but indicating some of the concrete exhibited problems with entrained air, some was abused by finishers, some was obviously not cured and much of it had little obvious problem other than scaling.

Our concrete was a combination of machine placed (slump <1.5") and hand placed (slump 3"-5"), has 15% to 20% fly ash (in part due to ASR problems) & 6%-8% entrained air. Bleeding progress in our dry climate is often difficult to assess.

Our interpretation of sporatic/inconsistent scaling:
**High air content, combined with troweling may have 'sealed' the surface on some of the hand placed concrete, resulting in large amounts of scaling or even delamination.
**The fly ash was SUBSTITUTED for cement, resulting in relatively low amounts of early heat of hydration, coupled with marginal to no curing & low air temperatures (below 40 degrees F) resulted in low early concrete strengths which did not really improve until the following Spring. Early Spring storms resulted in surface moisture which may or may not have frozen, but surface scaling began.
**Improper sealing/curing materials used or placed too soon which prohibited the removal of moisture from the concrete mass, probably concentrated this water near the surface which froze in the winter, producing scaling in the Spring.

 

[beton1]

You need to some testing. All the replies are in line, and, very good. You do not state if there was any initial testing, ie. cylinders taken for compressive strength, or field air content testing. You have to establish if the concrete 'claimed quality' is what was specified. The concrete strength has to be verified and the petrographic analysis will reveal what is going on with the air content/distribution, particularly at the surface. I am curious, since there appears to be exterior/interior concrete, is the work residential; such as sidewalks, driveways and garages?

 

[jryooper]

beton1,

Yes, the concrete with the "issues" is residential (sidewalks, drives, and garages). No initial testing was done for any of the placed concrete. I know I need to have some testing done on these slabs, but the expense of doing that seems unrecoverable and is hardly in the budget right now. Any more information you could pass to me would be greatly appreciated. Thanks a lot.

 

[beton1]

I still emphasize that testing needs to be done. You may want to do a preliminary test to get a handle on the problem, prior to full scale testing. How do you tell the finishers, with '30 years experience', that they are at fault without some back-up. At least try to rule out the concrete quality (delivered); Lafarge may co-operate as they have there own testing labs (at least in Ontario). If the areas in question are a small percentage, it may be cheaper to do the repairs; something eventually will be done? Hopefully not replacement. If you are the consultant or owners rep, you are at one vantage point, trying to get what you expected. If you are on the other side, contractor, you are trying to get paid and may also be looking for relief from the subcontractor (finishers) or the ready-mix producer. I think you have enough advice on the technical part, the rest only you can answer. There is no cheap solution.

 

[concretemasonry]

jryooper -

In your area (I guessed/assumed from your name - I lived in the lower upper) there is substantial freeze-thaw exposure, but possibly not as bad as areas to the south of you that actually may get more cycles.

Freeze/thaw damage is a result of the cycles the concrete has been subjected to. That is why all durability tests are subjected to the ASTM standards (there are several, depending on the type of concrete and exposure) that prescribe the conditions, water type, number of cycles and amount of permissible loss. The key is the number of cycles since no sane person would expect an immediate failure. - Just try to schedule a freeze/thaw test series and you will find out how important the duration and cycles are. It takes time to measure durability since the cycles are important.

There are some general perameters that may give some indication of concrete durability (aggregate durability tests, concrete compression strength and absorption), but they are only indicators because the actual concrete is dependent on the combination of the factors working together or against each other through many cycles. Just as in a chain, the weakest link causes the failure that can be progressive.

As an extreme example, I have seen 3500 psi concrete with high absorption out-perform 8000 psi low absorption concrete by a large margin.

You can do everything correct about the mix design, batching, mixing time, slump, air entrainment, placement, but if the finishing is not correct AND timely, the weak link may be the final uncontrolled step - finishing.

Dick